1. Field of the Invention
This invention relates to gas turbine engines and more particularly to engines having nozzle guide vanes which are both rotatable and coolable at the inlet to the turbine.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, pressurized air and fuel are burned in a combustion chamber to add thermal energy to the medium gases flowing therethrough. The effluent from the chamber comprises high temperature gases which are flowed downstream in an annular flow path through the turbine section of the engine. Nozzle guide vanes at the inlet to the turbine direct the medium gases onto a multiplicity of blades which extend radially outward from the engine rotor. The nozzle guide vanes are particularly susceptible to thermal damage and are commonly cooled to control the temperature of the material comprising the vanes in the face of high turbine inlet temperatures. Cooling air from the engine compressor is bled through suitable conduit means to an annular chamber which is located radially outward of the working medium flow path. The platforms of the nozzle guide vanes in conventional constructions separate the cooling air in the chamber from the working medium gases in the flow path.
Recent efforts to improve the performance of gas turbine engines have shown that variations in the turbine nozzle area under diverse operating conditions is advantageous. In most constructions the area variation is accomplished by rotating the nozzle guide vanes at the inlet to the turbine. A typical rotating vane construction is shown in U.S. Pat. No. 3,224,194 to DeFeo et al entitled "Gas Turbine Engine". In DeFeo et al the rotatable vane, which is supported at both the internal and external walls of the medium flow path, is also coolable. Cooling air is supplied to the vane through air chambers and conduits internally of the medium flow path. A plurality of shroud segments each having an hourglass shape separate the air chambers from the medium flow path. The rotatable vane extends between each pair of adjacent shroud segments and into the flow path with sealing means being provided at the upstream and downstream ends of the shroud segments but not at their interfaces with the rotatable vanes. Under divergent thermal conditions wherein the temperature of the shroud material varies from that of the shroud supporting structure, the gap between adjacent shroud segments becomes excessive and allows the wasteful leakage of cooling air therethrough.
Inasmuch as the modern turbines which employ the described rotatable vanes are operated at very high inlet temperatures, the thermal divergence of the components is substantial and improved means for containing the cooling air under these conditions is required.